1. Technical Field
The disclosure relates to a driver, and more particularly to a transmission line driver and a method for driving the same.
2. Related Art
A common transmission line driver is a voltage buffer, which drives a loading resistor by providing an output current, in which an impedance-matching resistance value may change according to a length of a driven cable, work environment, or other external factors. In addition, in order to drive the loading resistor effectively, the transmission line driver must maintain a larger output voltage, a higher linearity, a better power efficiency, and a better impedance matching of the transmission line.
Two types of conventional modes for driving the transmission line exist, a voltage source driving mode and a current source driving mode. In current source driving technology, a current signal is used to push a load impedance RL of the transmission, so as to generate a sufficient amplitude of an output voltage, such that the transmission line may transmit a signal.
FIG. 1 is a circuit diagram of a transmission line driver 100 driven by a current source. The transmission line driver 100 includes an amplifier 101, a current source 102, a transistor 103, an impedance matching resistor Rs, a loading resistor RL, and a feedback resistor Rf. The feedback resistor Rf is coupled to the current source 102. The transistor 103 has one end coupled to a circuit voltage source VDD and the other end coupled to the other end of the feedback resistor Rf. The amplifier 101 has an input end coupled to the feedback resistor Rf and the current source 102 and an output end coupled to a gate of the transistor 103. The impedance matching resistor Rs is coupled between the feedback resistor Rf and the loading resistor RL. The impedance matching resistor Rs theoretically matches the loading resistor RL. Otherwise, problems of signal reflection and power consumption occur easily. It should be noted that the current source 102 is an internal current source, and is generated by providing a voltage for an internal resistor.
During production process, the circuit in FIG. 1 may incur problems due to process variation when an integrated circuit (IC) is manufactured. During the manufacturing of the IC, the production process of the common resistor may have a variation possibility of 20%. This variation may result in a corresponding variation of the impedance matching resistor of the transmission line driver, such that the output voltage may be changed to a certain degree. In other words, the values of the feedback resistor Rf and the impedance matching resistor Rs are different from the designed values due to the process variation, resulting in the problem that the output impedance is changed and the output voltage is unstable.
FIG. 2 is a circuit diagram of a transmission line driver 200 driven by a current source. The transmission line driver 200 includes an amplifier 101, a current source 102, a transistor 103, an adjustable impedance matching resistor Rs, a loading resistor RL, and a feedback resistor Rf. After production the adjustable impedance matching resistor Rs is adjustable to match the loading resistor RL, such that the output voltage Vout is fixed. Meanwhile, a value of the adjustable impedance matching resistor Rs may be fixed through the calibration. However, the mode increases production costs. It should be noted that the current source 102 is an internal current source, and is generated by providing a voltage for an internal resistor.
FIG. 3 is a circuit diagram of a transmission line driver 300 driven by a current source, in which another solution is shown. The transmission line driver 300 includes an amplifier 101, a current source 102, a transistor 103, an adjustable impedance matching resistor Rs, a loading resistor RL, and an adjustable feedback resistor Rf. After production, the adjustable impedance matching resistor Rs is adjustable to match the loading resistor RL, such that the output voltage Vout is fixed. Meanwhile, a value of the adjustable impedance matching resistor Rs may be fixed through the calibration. In addition, the adjustable feedback resistor Rf is adjustable, such that output of a reference voltage Vpp is fixed, and the output of the Vout may further satisfy a technical demand. However, the mode also increases production costs. It should be noted that the current source 102 is an external current source, generated by providing a voltage for an external resistor.
Thus in the prior art, the output voltage Vout is fixed and the output impedance is fixed through the calibration of the impedance matching resistor. Although the problems of impedance matching and the outputting fixing may be solved, the cost is high. In addition, it is not practical to produce a circuit which completely matches design preferences, which is the major point of consideration in the design of the transmission line driver in order to obtain the optimal cost performance ratio, that is, to satisfy product quality demand and ensure the lowest production cost. For example, in certain application products, the impedance matching allows an error of 20% and the voltage change allows an error of 5%, so that the product design with the lowest cost is more competitive, according to this standard.